1. Field of the Invention
The invention relates generally to the field of wellbore instruments and well logging methods. More specifically, the invention relates to systems and methods for communicating data and commands between wellbore disposed instruments and a surface disposed control and recording unit.
2. Background Art
Well logging instruments are devices configured to move through a wellbore drilled through subsurface rock formations. The devices include one or more transmitters, sensors, actuators, and other devices that measure various properties of the subsurface rock formations and/or perform certain physical or mechanical acts on the formations, such as drilling or percussively obtaining samples of the rock formations, and withdrawing samples of connate fluid from the rock formations. Measurements of the properties of the rock formations made by the sensors may be recorded with respect to the instrument axial position (depth) within the wellbore as the instrument is moved along the wellbore. Such recording is referred to as a “well log.”
Well logging instruments can be conveyed along the wellbore by extending and withdrawing an armored electrical cable (“wireline”), wherein the instruments are coupled to the end of the wireline. Such conveyance relies on gravity to move the instruments into the wellbore. Extending and withdrawing the wireline may be performed using a winch or similar spooling device known in the art. It is also known in the art to use “logging while drilling” (“LWD”) instruments in certain circumstances. Such circumstances include expensive drilling operations, where the time needed to suspend drilling operations in order to make the wellbore accessible to wireline instruments would make the cost of such access prohibitive, and wellbores having a substantial lateral displacement from the surface location of the well. Such circumstances include large lateral displacement of the wellbore particularly where long wellbore segments having high inclination (deviation from vertical). In such cases, gravity is not able to overcome friction between the instruments and the wellbore wall, thus making wireline conveyance impracticable unless it is complemented by an actuator, known as a tractor, to provide movement. LWD instrumentation has proven technically and economically successful under the appropriate conditions. LWD instrumentation is also typically used in conjunction with so-called measurement while drilling (“MWD”) instrumentation. MWD instruments typically include navigation sensors that determine the geodetic trajectory of the wellbore, and can include sensors that measure mechanical parameters such as torque applied to the pipe string, vibration, angular acceleration, and pressure in the annular space between the wall of the wellbore and the drill string.
MWD and LWD instruments are typically disposed in thick walled segments of pipe called drill collars, and such collars are typically disposed proximate the lower end of a “string” of pipe segments coupled end to end to form a “drill string.” Signals from the various sensors in MWD and LWD instruments are typically communicated to the Earth's surface using a device that modulates flow of drilling fluid as the fluid is pumped through the drill string. One or more pressure transducers disposed at the surface detect pressure changes caused by the modulator. The detected pressure changes are then decoded into the signals that were transmitted by the MWD and LWD instruments. Mud flow modulation telemetry is typically limited to a transmission rate of several bits per second.
Another MWD/LWD data communication technique known in the art of extremely low frequency (“ELF”) electromagnetic telemetry. Just as is the case with mud modulation telemetry, the data rate is relatively low. Further, electromagnetic telemetry is not useful where subsurface formations are highly electrically conductive.
More recently, a type of drill pipe has been developed that includes an electromagnetic signal communication channel. See, for example, U.S. Pat. No. 6,641,434 issued to Boyle et al. and assigned to the assignee of the present invention. Such drill pipe has in particular provided substantially increased signal telemetry speed for use with LWD instruments over conventional LWD signal telemetry, which typically is performed by mud pressure modulation or by very low frequency electromagnetic signal transmission.
In any of the foregoing examples, as well as other types of well logging instrumentation, data obtained from the various sensors in the MWD and LWD instruments are communicated to a device such as a data acquisition and recording unit disposed at the Earth's surface. Data and command signals may also be communicated from the device to the various instruments in the wellbore. Even when using multiple data communication systems, e.g., both wired drill pipe and mud flow modulation (or electromagnetic), the volume of data generated by typical MWD/LWD instrument configurations is such that optimized use of the data communication channels is desirable. The present invention addresses various forms of data communication to optimize use of available communication channels.